The present invention relates to a method of determining rotational speed, size and instantaneous direction of a residual flux of an inverter-supplied squirrel cage induction machine, to be used for starting the squirrel cage induction machine when there exists a rotating residual flux in the machine and a total leakage inductance of the machine is known.
The method of the invention can be used for instance for inverter drives, in which a restarting command can turn up after the operation has been stopped, before the flux of the squirrel cage induction machine has disappeared. The invention can further be used for a quick recovery from disturbances, such as mains interruption or inverter overcurrent release. The method can also be used when a squirrel cage induction motor is connected from a mains supply to an inverter.
EP Patent 469 177 discloses a method according to the prior art, by which a squirrel cage induction machine is started in a situation when there exists a rotating residual flux in the machine. In the method described in this publication, the computing time is at least twice the period of a basic cycle of the residual flux, and after the rotation has been measured, a magnetization current of the squirrel cage induction machine shall be raised to the nominal value before changing over to a controlled operation. Accordingly, the starting provided by this method is relatively slow and a direct change-over to a controlled supply of the squirrel cage induction machine is not possible at this starting.
The object of the present invention is to provide a method by means of which the residual flux rotating in the squirrel cage induction machine can be determined in such a way that a voltage feed to the motor, synchronized with a residual magnetic flux therein, can begin immediately after the determination of the flux. The method must not be dependent on how the rotating flux has been generated in the squirrel cage induction machine before the machine is started by inverter supply.
The object of controlling an asynchronous machine is generally to cause a desired behaviour of a moment-generated by the machine, when the current and the voltage fed to the machine are known. The purpose is then to influence the electric moment, the relative value of which as a function of a stator flux and current is: EQU T.sub.m =c(.PSI..sub.s .times.i.sub.s) (1)
where:
T.sub.m =electric moment, PA1 c=constant coefficient, PA1 .PSI..sub.s =stator flux, and PA1 i.sub.s =stator current. PA1 i.sub.r =rotor resistance, PA1 .omega..sub.m =mechanical rotational speed, PA1 R.sub.r =rotor resistance, PA1 R.sub.s =stator resistance, PA1 L.sub.s =stator inductance, PA1 L.sub.r =rotor inductance, and PA1 L.sub.r =main inductance.
Consequently, a controlled moment control requires that, in addition to the current, the stator flux of the machine or a magnitude proportional to that (such as rotor flux or air gap flux) is known. For starting a squirrel cage induction machine it is desirable that a starting to the rotating residual flux can take place as quickly as possible, without moment shocks and current peaks.
The generally known differential and current equations of the stator and rotor of a squirrel cage induction machine in a coordinate system of the stator are: ##EQU1## where: .PSI..sub.r =rotor flux,
Further, it is possible to derive from the equations 4 and 5 ##EQU2## where: ##EQU3## =leakage coefficient.
When control pulses are eliminated from inverter switches, i.e. an individual switch is not connected to an upper or a lower branch of the inverter but is left totally unconnected, the stator circuit of the squirrel cage induction machine is switched off and the stator current is set to zero after a very quick transient. After the control pulses have been quenched, the stator current can flow via idle current diodes of the inverter switches only. After this the stator flux is maintained by the rotor current alone. After unwinding the stator winding, the stator flux follows the equation: ##EQU4## where
.PSI..sub.ro =rotor flux at unwinding moment of stator winding,
.tau..sub.r =L.sub.r /r.sub.r, i.e. time constant of rotor,
.omega.=residual flux frequency, and
t=time.
A decreasing residual flux does not generate a moment and the stator and rotor flux have the same phases: ##EQU5##